1. Field of the Invention
This invention relates to a Hall accelerator having a discharge area with anode and cathode means and an electromagnet producing a magnetic field therein, which accelerator causes gas discharge therein when a gas is injected thereto so as to generate, accelerate and emit Hall ions therefrom. More particularly, the invention relates to a Hall accelerator with preionization discharge, which accelerator has a magnetron type anode means adapted to produce an auxiliary discharge thereat.
2. Description of the Prior Art
The Hall accelerator is an effective source of beam ions for ion implanting operations in semiconductor device production and in the improvement of metallic material properties. The ever increasing demand for semiconductor devices is expected to continue in the future. However, there are various limitations in the Hall ion beam produced by Hall accelerators of the prior art; namely, a comparatively large maximum current of up to one kA, a low voltage in the order of several hundreds of volts, and a comparatively wide divergence angle of about 30 degrees in case of short pulses in the order of several milliseconds.
FIG. 10 illustrates a Hall accelerator of the prior art. A discharge area with an annular cross section is defined between concentric cylindrical insulating walls IW, which discharge area is provided with a disk-shaped anode A mounted on one end thereof and a cathode C mounted on the opposite end thereof. The cathode C has a large circular opening aligned with the anode A. Iron cores F in the form of shell type with a solenoid S wound on its central leg are disposed in contact with the insulating walls IW in such a manner that a radial magnetic field is produced in the annular cross-sectional space of the discharge area. A suitable gas is directly introduced into the discharge area through a gas valve GV, so as to cause gas discharge therein for emitting ions. The Hall accelerator of such simple arrangement has a shortcoming in that, due to the heat generation at the electrodes, the ion emission is restricted to be of low voltage and short pulse duration even if a large current is possible, and a wide beam divergence angle is inevitable.
On the other hand, practical requirements for Hall accelerators from the above-mentioned industrial treatments of semiconductor and metallic materials and from nuclear fusion and other research activities demand continuous and reliable ion beams with high voltages of several kV to several hundred kV and medium currents of several A to several ten A at a small divergence angle.
To meet such requirements, the Culham Research Laboratory of England has developed an improved Hall accelerator as shown in FIG. 11. In the figure, two concentric quartz walls Q, i.e., an inner cylindrical wall and an outer tubular wall, define an elongated discharge area with an annular cross section. An annular tungsten anode TA and an annular copper cathode CC are disposed at opposite ends of the discharge area in a manner similar to that of FIG. 10. A long shell type iron core F carrying a first stage solenoid S.sub.1 and a second stage solenoid S.sub.2 is in contact with the quartz walls Q in such a manner that a radial magnetic field is generated in the discharge area. Thereby, the Hall accelerator of FIG. 11 emits a continuous Hall ion beam with a maximum voltage of up to 30 kV and a maximum current of up to 1.5 A for several seconds.
Although the voltage and current requirements are met by the above development, performance relating to the beam divergence angle and the stability of the Hall accelerator of FIG. 11 has not been reported yet. The direct introduction of gas into the discharge area in the last mentioned Hall accelerator implies that the conventional problems, such as the gas efficiency, the beam divergence angle, the reproducibility, the operable regions, and the like, might not have been fully solved. Besides, most of the Hall accelerators of this type have been used for pulse-like operation, and factors affecting continuous operation, such as the cooling of the electrodes in the discharge area, have not been fully considered as can be seen from the structure of FIG. 10.
The example of FIG. 11 cools the electromagnet with cooling oil and the copper cathode CC with cooling water by using coolant inlets WI and coolant outlets WO. The anode TA in the discharge area is, however, simply made of tungsten having a high melting point but not cooled. With the uncooled annular anode TA, any continuous operation for a long period of time will be difficult, because the annular anode has only a small surface area and it will be heavily bombarded by high-density electrons generated by gas discharge in the discharge area. Thus, the Hall accelerators of the prior art have a shortcoming in that long continuous operation is difficult due to the lack of anode cooling and that water cooling for the anode at a high voltage poses a structural difficulty from the standpoint of electric insulation.